(1) Field of the Invention
The present invention relates to a duplex stainless steel seamless pipe and a method for producing the same, and more particularly, relates to a method capable of producing inexpensively a duplex stainless steel seamless pipe in high productivity and yield.
(2) Description of Prior Art
Duplex stainless steel represented by JIS SUS 329J1 has high resistances against corrosion, stress corrosion cracking and grooving corrosion, and a high weldability, and has been noticed as a material for piping in various chemical plants, for pipe for oil well, for pipe for subterranean, for line pipe and the like.
Stainless steel seamless pipe is generally produced by plug mill process, mandrel process, pilgar mill process, Ugine.Sejournet process, Ehrhardt-Verfahren mill process and the like.
Duplex stainless steel has a ferrite-austenite duplex texture, and is poor in hot workability. Therefore, hot extrusion methods such as Ugine-Sejournet process and the like have hitherto been used in the production of seamless pipe from stainless steel having a poor hot workability.
However, in the direct piercing method by a hot extrusion method, when the length of a billet becomes as large as 5-7 times the diameter thereof during the piercing, the deviation of the wall thickness of the billet becomes large, and hence it is difficult to produce a continuous length pipe. In order to solve this problem, a continuous length pipe is produced by a so-called expansion method, wherein a hole has previously been bored through a billet at its center by the machining, and the bore is expanded. However, even in this expansion method, the length of a billet is limited to not more than about 15 times the diameter of the billet.
In the Ugine-Sejournet process, a vitreous lubricant is used, and therefore a step for peeling the vitreous lubricant from a rolled hollow piece is necessary. This step is a troublesome step.